As a method of detecting a test substance (i.e., a bio-related substance contained in a sample), a known method is, for example, a method comprising: using a test substance, a specific binding substance which specifically recognizes the test substance, and metal particles; capturing in a neighborhood of a surface of a working electrode the metal particles in an amount corresponding to an amount of the test substance by using biological interactions (e.g., antigen-antibody reaction); and electrochemically measuring the amount of the captured metal particles (for example, refer to U. S. Patent Application Publication No. 2009/159458, Japanese Patent No. 4915740, Koutarou Idegami et al., “Gold Nanoparticle-Based Redox Signal Enhancement for Sensitive Detection of Human Chorionic Gonadotropin Hormone”, Electroanalysis, vol. 20, pp. 14-21, published on Sep. 27, 2007, and Miyuki Chikae et al., “Highly Sensitive Method for Electrochemical Detection of Silver Nanoparticle Labels in Metalloimmunoassay with Preoxidation/Reduction Signal Enhancement”, Electrochemistry, vol. 78, pp. 748-753, published in 2010, all of which are incorporated herein by reference).
U. S. Patent Application Publication No. 2009/159458 and Koutarou Idegami et al. disclose a method of detecting a test substance, comprising the steps of: capturing in a neighborhood of a surface of a working electrode gold particles in an amount corresponding to an amount of the test substance by using biological interactions; electrochemically oxidizing the captured gold particles to be eluted; and measuring a current induced by reducing the eluted gold ions. Japanese Patent No. 4915740 or Miyuki Chikae et al. describes a method of detecting a test substance, comprising the steps of: capturing in a neighborhood of a surface of a working electrode silver particles in an amount corresponding to an amount of the test substance by using biological interactions; electrochemically oxidizing the captured silver particles to be eluted; reducing the eluted silver ions to deposit the silver on the surface of the working electrode; and measuring a current induced by electrochemically oxidizing the deposited silver.